1. Field of the Invention
The present invention relates to an overhead projector for projecting enlarged images onto a screen from a transparent original such as a transparency or a liquid crystal panel.
2. Description of the Prior Art
Overhead projectors (hereinafter also referred to as OHPs) are widely used in conferences and seminars to visually present charts, graphs, and the like to the audience concurrently with oral explanations. An OHP projects enlarged images onto a screen from a transparent original such as a transparency or a liquid crystal panel. To achieve this, an OHP is provided with a projection lens for projecting images onto a screen from a transparent original, and a reflection mirror for reflecting the light from a light source toward the screen. Inconveniently, however, these components are usually arranged above the body of the OHP by the use of support members, and thus they often obstruct part of the view of the images projected onto the screen.
To overcome this inconvenience, various attempts of improvement have been made to OHPs. For example, Japanese Laid-open Patent Application No. H4-366938 discloses a "transmitting OHPT". In this OHP, which allows oblique projection of images, the projection lens and the reflection mirror are arranged to have their optical axes off the center line of the stage for placing an original toward the screen, and a Fresnel lens is arranged to have its optical axis at the intersection between the line from the light source to the projection lens and the surface of the Fresnel lens. As a result, this OHP can project images free from trapezoid distortion onto a screen whose base line is approximately at the same vertical position as the reflection mirror. On the other hand, Japanese Laid-open Patent Application No. H4-247441 Discloses a "reflecting OHP", and this OHP, too, allows oblique projection of images.
Moreover, Japanese Laid-open Patent Application No. H6-160977 discloses another type of OHP that is based on a "reflecting OHP" but does not have any supporting member as mentioned above. This OHP consists of a projector body including a light source, a first reflection mirror for changing the path of light, a projection lens, and a stage for placing a transparent original, and a cover including a transmitting Fresnel lens that is pressed directly onto the original stage, and a second reflection mirror. In this OHP, the light from the light source is reflected by the first reflection mirror onto the image on the transparent original, and the light therefrom is condensed by the second reflection mirror and the Fresnel lens that are mounted on the cover, is then reflected by the first reflection mirror into the projection lens, and is then projected therefrom to form an enlarged image of the original.
However, the OHPs disclosed in Japanese Laid-open Patent Applications Nos. H4-366938 and H4-247441 still have their projection lens and reflection mirror arranged above the projector body by the use of support members. Accordingly, these OHPs provide an unobstructed view of the projected images only within limited ranges of viewing angles, and need to be used in a room where it is possible to secure sufficiently wide space and a sufficiently high ceiling. On the other hand, the OHP disclosed in Japanese Laid-open Patent Application No. H6-160977 is not user-friendly, because its top cover needs to be opened and closed every time the transparent originals are exchanged.
Moreover, most of conventional OHPs employ, as a condenser lens for condensing the light from the image on the transparent original, a transmitting Fresnel lens that has fine concentric grooves formed on its surface. Such a Fresnel lens, however, causes a considerable loss of light due to scattering or other of light, and thus its use results in insufficient brightness of the images projected onto the screen. Brightness can be increased by increasing the power of the light source, but this leads to higher cost, because, it is then necessary to take some measures against accordingly increased generation of heat.